The present invention relates generally to gas turbine engines. More particularly, the present invention relates to a method for controlling thermoacoustic instabilities in a combustor.
Thermoacoustic instabilities arise in gas turbine and aero-engines when acoustic modes couple with unsteady heat released due to combustion in a positive feedback loop. These instabilities can lead to large pressure oscillations inside the combustor cavity, thereby affecting its stable operation and potentially causing structural damage to the combustor components. Two particular examples of thermoacoustic instabilities in annular combustors are the “screech” instability in the afterburner and the “howl” instability in the primary combustion chamber.
Prior art approaches for control of thermoacoustic instabilities typically utilized passive liners or tuned resonators configured to damp the acoustic mode. However, these solutions suffer from several disadvantages. In particular, they introduce additional weight and may be expensive to implement. In addition, resonators are effective only over a limited range of frequencies and become ineffective if frequency of the instability changes because of, for example, changes in operating conditions. These passive devices have to be cooled, which may detrimentally affect the efficiency of the engine. Finally, effective tuned resonator design requires a prior knowledge of the frequency of instability.
Active combustion control has also been considered as an approach for control of thermoacoustic instabilities. Active approaches usually require an accurate mathematical model of the thermoacoustic dynamics for control design. However, on account of complex combustion physics, the exact physical mechanism underlying the initiation and sustenance of instabilities such as screech typically is not understood. Furthermore, there are implementation issues such as lack of suitable bandwidth fuel valves that are needed for active control.
The thermoacoustic instabilities typically appear only during a small portion of an aero-engine's flight envelope or operating conditions in the case of land-based combustors. Thus, passive dampers and active control systems are useful to help control thermoacoustic oscillations only over a small portion of operating conditions and have no useful function at nominal operating conditions. Furthermore, they negatively affect weight and performance of the engine at the operating conditions where the instability is not present.